Well, Negan's end in the comics isn't straightforwardly due to one specific killing act. It's more complicated and involves multiple factors and characters' actions.
Well, Negan first appears in the series as a major antagonist. He has a very distinct personality. He's confident, often to the point of being cocky. His group, the Saviors, would extort other communities for resources. He uses fear as a tool to control those around him. His actions have a huge impact on the other characters in the show, like Rick and his group.
Well, it depends on your perspective. But generally, Negan is considered bad in the comic because of his cruel and violent actions towards the main characters.